1. Field of the Invention
This invention relates generally to power supplies, and more specifically, to a switched mode power supply.
2. Background Information
In many electronic device applications, especially the low power off-line adapter/charger market, an approximately constant voltage/constant current output characteristic is required. Known switched mode power supply circuits providing constant output current and voltage characteristics typically use circuitry on the secondary (or output side) of the power supply that sense output voltage and current and generate a feedback signal. The feedback signal is typically communicated to a regulator circuit on the primary side of the power supply through an opto coupler component. This feedback signal is then used to control the switching of a primary switch to provide the required power supply output characteristic.
The specific function of maintaining constant output current is normally achieved with secondary current sense resistors in series with the output load that provide a voltage signal proportional to the current flowing through the secondary current sense resistors. This voltage signal is then used to provide a feedback signal, which in turn is used to control the power switch.
Another technique to provide an approximately constant output current is to derive information about the output voltage of the power supply from an auxiliary winding of the power supply transformer on the primary side of the power supply. This information is then used to adjust the primary switch current limit threshold independent of any supply current required by the primary regulator circuit.